1. Field of the Invention
This invention relates to a control system for a flash device of a camera.
2. Description of the Related Art
The known flash adjusting methods heretofore employed for flash devices of cameras of the kind using a silver-halide film include an external-metering flash adjusting method and a TTL flash adjusting method.
In accordance with the external-metering flash adjusting method, the flash device is arranged as follows: The flash device is provided with an external-metering flash adjustment sensor. Light reflected from an object to be photographed as a result of flash light emission by the flash device is received and temporally integrated by the external-metering flash adjustment sensor. When an integrated value of the reflected light reaches an apposite light quantity which is determined according to a film speed and an aperture value of the camera which have previously input to the flash device, the flash light emission by the flash device is brought to a stop. It is an advantage of this method that the flash adjustment can be carried out without having any additional device arranged within the camera for that purpose. However, this method has the following disadvantages: There arises some parallax between a photo-taking lens of the camera and the flash device. Since a light receiving angle of the external-metering flash adjustment sensor is fixed, it is impossible to cope with a change in the angle of view. There is a great influence of a background of the object. In accordance with this method, therefore, it is difficult to make the flash adjustment with a high degree of accuracy.
In the case of the TTL flash adjusting method, an image of the object which is obtained by illuminating the object with a flash device and which is projected on the surface of film is measured in light by means of a light receiving element which is disposed within the camera. The flash light emission by the flash device is brought to a stop when the quantity of light received by the light receiving element reaches an apposite level which is preset at the flash device on the basis of a film speed and an aperture value of the camera. This method permits the flash adjustment to cope with a change in the angle of view without being affected by any parallax. The flash adjustment thus can be carried out with a high degree of accuracy. Therefore, in these days, the TTL flash adjusting method is most popularly employed for a single-lens reflex camera. However, a shortcoming of that method lies in that: Since light reflected from the film surface which is very weak is to be measured, a difference in reflection factor among different film products and the influence of a background of the object which varies depending on the position and the size of the object tend to result in a flash adjustment error.
In addition to the above-stated external-metering flash adjusting method and the TTL flash adjusting method, there has been known another flash adjusting method which is the most fundamental method and which is called a flashmatic method. The flashmatic method (hereinafter referred to as DV (distance value) flash adjusting method) is as follows: An aperture value is determined for the camera by dividing the guide number (GNO.) of the flash device by information on a distance to the object which is obtained either from the flash device or from the camera. After that, the flash device is allowed to emit a flash light for an apposite exposure.
The DV flash adjusting method permits highly accurate flash adjustment without being affected by the position, the size and the background of the object. However, in a case where the flash device is arranged to have the flash adjustment automatically carried out, the DV flash adjusting method necessitates the use of highly accurate distance measuring means with the flash adjustment arranged to be dependent on the object distance information obtained from the camera, i.e., obtained from a distance ring when the photo-taking lens is in an in-focus state, because: If the range finding (distance measuring) device of the camera is of a passive type, the result of distance measurement tends to be inaccurate in a specific condition of the object. The distance measurement error increases particularly in the event of a short focal length of the photo-taking lens. Besides, the error also increases accordingly as the shooting distance to the object increases.
Meanwhile, cameras of the kind arranged to convert an optical image of an object into an electrical signal by means of an image sensor such as a CCD or the like without using any silver-halide film (hereinafter, such a camera being referred to as an electronic still camera) have recently been developed. The accuracy of the above-stated flash adjustment must be higher for the electronic still camera than for the camera of the kind using the silver-halide film, because the image sensor which is a CCD or the like has a narrower latitude than the silver-halide film. For example, the allowable flash adjustment error in the electronic still camera is within a range of .+-.1 EV while that in the silver-halide film (negative film) is .+-.5 EV.
With such a high flash adjustment accuracy required by the electronic still camera, the TTL flash adjusting method is more suitable for the electronic still camera than the external metering flash adjusting method as the former is more accurate than the latter. Assuming that the TTL flash adjusting method is applied to the electronic still camera, light reflected from the object is receivable by one of the following two conceivable methods:
(I) A method of providing a half-reflection mirror within a photo-taking optical path and arranging a light receiving sensor to receive the object's light reflected from the half-reflection mirror; and PA1 (II) a method of receiving the light reflected from an image forming plane of the CCD by means of a light receiving sensor.
However, in the case of the method (I), the half-reflection mirror arranged within the photo-taking optical path reduces the quantity of light incident on the image forming plane of the CCD. This lowers the sensitivity of a photographing operation of camera. Further, the addition of the half-reflection mirror is a spatial disadvantage.
In accordance with the method (II), the light reflected from the CCD is very weak and available only in a small amount as the image forming plane of the CCD is provided with an anti-reflection coating or the like. This makes the flash adjustment difficult.
In a case where the external metering flash adjusting method is applied to the electronic still camera with some means for increasing the accuracy of flash adjustment, this method necessitates a parallax structural arrangement and some device for controlling a light receiving angle of the light receiving sensor. This requirement results in a complex arrangement of the flash device and thus increases the size of the flash device.
Meanwhile, adoption of the DV flash adjusting method would be disadvantageous for the electronic still camera in terms of the distance measuring accuracy because the flash adjustment must be by carried out for more accurately for the electronic still camera than for the silver-halide film camera and because a photo-taking lens of the short focal length is more likely used for the electronic still camera than for the silver-halide film camera since the angle of view of the electronic still camera is little as compared with that of the silver-halide film camera.